This invention relates to both a low distortion amplifier and to a method of achieving low distortion in an amplifier.
This invention has particular application to amplifiers whose power output stage intrinsically produce most distortion at low frequencies, and in particular, at audio frequencies.
There has been considerable human effort into attaining low distortion in amplifiers of many applications at all frequencies. In 1950, the best audio power amplifiers produced distortion of about 0.1% at 1 kHz, and in the 1990s, this has been reduced to about 0.001% at 1 kHz, and about 0.02% at 20 kHz, although one manufacturer claims 0.0025% at 20 kHz.
The vast majority of commercial audio amplifiers more or less follow well established standard designs.
There are some exceptions: a Technics SE-A1 amplifier which is known of in some countries incorporates an A-class output stage supplied by a floating low voltage high current power supply. This power supply is connected to a B-class high voltage output stage.
An LT1166 integrated circuit is primarily intended to control the quiescent bias feeding output transistors in audio amplifiers. The LT1166 consists of a low gain transconductance differential amplifier (gain of 0.125 mho) with an inverting and a non-inverting input. The circuitry has a local negative feedback path connecting an output of the power stage to the inverting input of the transconductance amplifier. The input of the output stage is the non-inverting input of the transconductance amplifier. Two local dominant poles necessary for stability are formed by the use of shunt capacitors to ground from the transconductance amplifier""s outputs. The Linear Technology application circuitry promises distortions no better than many commercial products.
In the Journal of Audio Engineering Society, vol 29, no 1/2, January/February 1981, pages 27-30, M. J. Hawksford, discloses as a mere paper publication a theoretical means of cancelling distortion in any amplifier stage, including an output stage. This is achieved by subtracting the signals feeding the output power transistors inputs from the amplifier output, and then adding this signal back into the signal driving the output transistors""inputs.
Iwamatsu in U.S. Pat. No. 4,476,442 again as a mere paper publication disclosed circuitry based on the principles of Hawksford. In one embodiment described in this patent specification, Iwamatsu discloses floating power supplies supplying the adding and subtracting circuitry. These floating supplies follow a voltage equal to the sum of the output signal plus a signal linearly proportional to current flowing through an output load. However, Iwamatsu""s circuits do not include local dominant poles.
Robert R. Cordell in xe2x80x9cMOSPOWER APPLICATIONSxe2x80x9d, Siliconix inc. ISBN 0930519-000, 1984, 6.6.3 discloses an audio power amplifier essentially the same as one of the Hawksford""s circuits, but including local dominant poles required for stability. This circuit has however no provision for thermal stability, nor floating power supply rails.
The current inventor Bruce H Candy previously in U.S. Pat. No. 5,892,398 as a paper publication only, disclosed an amplifier also utilising the principles of Hawksford, but including local dominant poles required for stability, thermal tracking circuitry for thermal stability, floating power supplies which track the output signal, rather than to the sum of the output signal plus a signal linearly proportional to the current flowing through the output load as in the case of Iwamatsu. Candy also disclosed an output stage input current source load which is also supplied by power from the floating power supplies. It was possible with such an arrangement according to my experiments which were not published to attain a distortion in the order of only 1 part per million at 20 kHz at several hundred watts of output.
Williamson et al. In U.S. Pat. No. 5,396,194 describes as a mere paper publication a switch mode amplifier containing floating low voltage high current power supplies which supply an A-class amplifier. This is similar to the technics SE-A1 except that the drive circuitry is switch-mode rather than class-B and that the power supplying the A-class amplifier is derived from the switch mode power supply rather than a separate power supply.
In one of the Williamson paper descriptions there was described floating power supplies to supply small signal operational amplifiers which were connected as servo loops to control the current flowing through the output devices. There are two feedback paths containing a capacitor which form two local dominant poles which are essential for stability.
The current inventor Bruce H Candy in U.S. patent application 09/054070 describes an amplifier consisting of at least one operational amplifier, a first error correction amplifier, connected up as a servo loop to control the output voltage, as opposed to the output current as in the case of Williamson et al. These operational amplifiers are supplied by power from floating power supplies which track the output voltage.
Candy further describes a local dominant pole required for stability, and the advantages of using wide band operational amplifiers, with gain bandwidth products of more than 100 MHz. In addition, I described a second error correction amplifier, consisting of another operational amplifier, also preferably wideband, connected up as a servo loop to control the output voltage stage which includes the first error correction amplifier. In other words, I described a 2nd order local dominant pole formed by the signal path being amplified by two error correction stages in series.
This also is supplied by the floating power supplies. I further described the advantages of implementing high gain stages with local negative feedback and the attendant local dominant poles required for stability in other stages of the amplifier to reduce distortion. This arrangement does not require the precise setting of the adding and subtracting electronics disclosed by Hawksford and related circuits.
Audio power amplifiers, or operational amplifiers, Usually consist of three definable stages, an input stage, voltage amplifier stage and output stage. In power amplifiers, the output stage, sometimes called the power output stage, usually produces most distortion. However, the distortion of the power output stage maybe substantially reduced by some of the concepts described herein.
Compared to these distortion reduced power output stages, the lowest distortion conventional input stages, voltage amplifier stages may produce substantially greater distortion.
Conventional low distortion input stages are usually a differential voltage to current converter which produce a differential output current. In these low distortion traditional architectures, the differential current output of this input stage is connected to a current mirror, and the output node of the differential current output of the input stage and current mirror is connected to a common emitter cascode amplifier; said common emitter amplifier sometimes being a Darlington. The amplifier""s dominant pole is set by a network including a capacitor connected between the output and input of this common emitter cascode stage. Details of these stages are described in a review by Douglas Self in a series of articles in xe2x80x9cElectronics World+Wireless Worldxe2x80x9d from August 1993 to January 1994, and also in his book, ISBN 0-7506-2788-3, xe2x80x9cAudio power amplifier design handbookxe2x80x9d, Newness, Reprinted 1997/1998, and the second edition ISBN 0 7506 4527 X, 2000. Another review is given by Ben Duncan, High Performance Audio Power Amplifiers, ISBN 0 7506 2629 1, Newness 1996.
An object of this invention is to provide a further circuit arrangement which assists in construction of even more accurate amplifiers or at the least, provides the public with a useful alternative.
In one form of this invention although this may not be the only or indeed the broadest form of this there is proposed an electronic amplifier having an input, and an output, and including an output stage containing output transistors being connected to the amplifier output,
the output stage including an output error correction stage containing a first amplifier and a second amplifier, an input to the output stage being connected to an input of the first amplifier,
wherein there are at least four local negative feedback paths,
a first local negative feedback path being between an output of the output stage and an input of the first amplifier,
a second local negative path being between an output of the first amplifier and an input of the first amplifier,
and a third local negative feedback path being between an output of the output stage and an input of the second amplifier,
and a fourth local negative path being between an output of the second amplifier and an input of the second amplifier,
an output of the first amplifier being connected to an input of output stage transistor buffers or output stage transistors through a first network,
an output of the second amplifier being connected to an input of output stage transistor buffers or output stage transistors through a second network,
such that the first network transfers high frequencies from the first amplifier to an input of output stage transistor buffers or output stage transistors more substantially than the second network, and the second network transfers low frequencies from the second amplifier to the to an input of output stage transistor buffers or output stage transistors more substantially than the first network, such that components of the first and second amplifier, first, second, third and fourth local negative feedback paths, first and second networks, and
output transistors and output stage transistor buffers, are selected to form a substantially second order local dominant pole,
and the amplifier having it""s input connected to an amplifier input stage,
wherein the input stage may include a current mirror and a voltage amplification stage, an output of the amplifier input stage being connected to the input of the output stage,
and the first and the second amplifier being supplied by power from a floating power supply means coupled to either an or the output of the output stage so that a voltage of floating power supply supplying the first amplifier and second amplifier will follow substantially an output voltage of the output stage.
Other embodiments of the invention are as defined in the attached claims.
In one preferred embodiment, the output stage contains two error correcting servo loops, a first and a second error correcting servo loop.
The first error correcting servo loop includes a first electronic amplifier, preferably a wideband amplifier, with an inverting and a non-inverting input.
The second error correcting servo loop includes a second electronic amplifier, preferably a wideband amplifier, with an inverting and a non-inverting input.
A first local negative feedback path connects an output of the amplifier to the inverting input of the first amplifier.
A second local negative feedback path is connected between the output of the first amplifier and it""s inverting input; the negative feedback path includes at least one capacitor, together with the first electronic amplifier set with a local dominant pole.
A third local negative feedback path connects an output of the amplifier to the inverting input of the second amplifier.
A fourth local negative feedback path is connected between the output of the second amplifier and it""s inverting input, which includes at least one capacitor, a second capacitor. The input to the error correction electronics, and thus the output stage, is at the first amplifier""s non-inverting input. The output of the first amplifier is connected to the second amplifier""s non-inverting input and to the input of the output transistors, or to the inputs of buffer amplifiers feeding the output transistors, via a first path including a first network, consisting of a high pass filter containing at least one capacitor, a third capacitor. The output of the second amplifier is connected to the input of the output transistors, or to the inputs of buffer amplifiers feeding the output transistors, via a second path including a second network which includes at least one resistor. The first path passes mostly, or only, high frequency signals to the inputs of the output transistors, or to the inputs of buffer amplifiers feeding the output transistors. The second path passes mostly or only lower frequency signals, including direct current signals, to the inputs of the output transistors, or to the inputs of buffer amplifiers feeding the output transistors. The components of the first and second amplifier, first, second, third and fourth local negative feedback paths, first and second networks, and output transistors and output stage transistor buffers, are selected to form a substantially second lower order local dominant pole. The power supplies supplying the first and second error correcting servo loops, consisting of the first and second amplifiers, first and second networks, first and second paths and first through to fourth local negative feedback paths, are floating power supplies which substantially track the amplifier""s output signal. Thus high frequency stability of the output stage is mostly determined by the first electronic amplifier and the first local dominant pole, and thus high frequency stability is relatively independent of the second electronic amplifier with it""s second local dominant pole. Thus the combination of the first and second error correcting servo loops creates a 2nd order local dominant pole as does the disclosure of Candy in U.S. patent application 09/054070, but herein the high frequency path does not include the second electronic amplifier as does the amplifier disclosed by Candy.
In order to successfully apply a reasonable amount of global negative feedback to the whole amplifier, and local negative feedback to the first and second error correcting servo loops, whilst maintaining a safe margin of stability, the frequency at which the closed loop phase shift exceeds 90 degrees of the whole output stage including the error correcting servo loops, should remain similar to that intrinsic to the output power transistors plus their buffers. Typical complementary voltage follower power MOSFET stages have useful responses up to a few MHz for unconditional stability. Above this frequency, the phase shift can substantially exceed 90 degrees, in which case at these frequencies, the open loop gain of any servo loop about which negative feedback is to be applied should be of the order of unity. Thus, the amplifiers employed in these servo loops should have gain-bandwidth products substantially more than a few MHz so that the intrinsic closed loop phase shifts of these amplifiers add little to the total phase shift. As xe2x80x9cvideoxe2x80x9d or xe2x80x9cwidebandxe2x80x9d operational amplifiers are now of low cost and common, these are suitable.
In the simplest case where the first local negative feedback path consists of a resistor, say 470 ohms, and the third local negative feedback path likewise, and the second local negative feedback path consists of a capacitor, say 150 pF, and the fourth local negative feedback path likewise, but 1 nF, and the first path first network) consists of a capacitor of say 2.2 nF, and the second path (second network) consists of a resistor of say 220 ohms, with the second and fourth local negative feedback paths closed, but the first and third open, the open loop gain of the servo loops at a few MHz is of the order of 1, and the amount of negative feedback at 20 kHz is a couple of thousand times. Thus the reduction in distortion of this stage may be three orders of magnitude.
In accordance with the teaching of this invention, an amplifier has been built that produces distortion harmonics to a 20 kHz sinewave of a few hundred parts per billion, that is of the order of xe2x88x92130 dB at several hundred watts output power.
The advantages of this arrangement compared to the arrangement disclosed by the current inventor Candy in U.S. patent application 09/054070 is two fold.
First, the phase shift at high frequencies is less, and hence, more global or local feedback may be applied. Second, the second electronic operational amplifier may have a more relaxed high frequency specification because the high frequency response delay is set by the lower supply voltage first operational amplifier, and the higher open loop gain operational amplifiers are more readily available with more relaxed high frequency specifications.
This error corrected output stage may produce substantially less distortion than conventional input stages and voltage amplifier stages. Hence to take full advantage of the low distortion of the error-corrected output stage disclosed herein, the other stages also need to be improved upon.
These stages may be substantially improved by the application of additional servo loops and local dominant poles forming networks required for stability. In particular, the distortion generated by conventional current mirrors and the voltage amplifier may be substantially reduced by local negative feedback.
A very low distortion current mirror may be implemented by passing the controlling current through a third resistor, across which a potential difference occurs according to Ohm""s law. This voltage is then fed to a high input impedance voltage to current converter. For example, the resistor through which the control current flows is connected to a first power supply. The input current end of this first resistor is connected to a non-inverting input of an operational amplifier. The operational amplifier""s output is fed to the control input of a transistor, preferably being a very low capacitance type with high gain. Two small signal wideband (RF) bipolar devices connected as a Darlington pair are suitable, with the input base as said input. The output emitter of this pair is connected to a fourth resistor and to the inverting input of said operational amplifier. The other end of the fourth resistor is connected to the first power supply. The collector outputs of the Darlington pair produces a current accurately in proportion to the first and second resistor ratio and in proportion to the control current. Note there is a high degree of local negative feedback. Assuming the operational amplifier is unity gain stable, then the local dominant pole required for stability is built within the operational amplifier. If said operational amplifier is a wide-band device, then the phase lag at high frequency will be relatively small, and thus have no substantial effect on the stability requirements of the amplifiers overall negative feedback. The recommended low capacitance requirement of the wideband Darlington will ensure minimal variable capacitance distortion as a function of variable voltage at high frequency.
A very low distortion voltage amplifier stage with a very high impedance virtual earth input may be implemented using high open loop gain local negative feedback with attendant local poles required for stability. For example, two amplifiers may be connected within the voltage amplifier stage, one may be used to correct for the base-emitter non-linearities of the voltage amplifier stage amplifying transistors, that is to ensure an accurate voltage to current conversion, and the other may be used both to increase the lower frequency open loop gain and also to implement a high input impedance.
In all three areas of substantially improved accuracy described herein, namely the power output stage, the current mirror and voltage amplifier stage, the closed loop phase shift at high frequency is approximately unchanged, but the distortion greatly improved compared to the conventional art. This is achieved by employing wideband-gain components with substantial local negative feedback whilst attaining stability by the implementation of local dominant pole forming networks. In each case, the local negative feedback corrects for nonlinearities of local transistors without affecting the intrinsic closed loop gain and phase shift of the stage, that is the closed loop gain/phase shift without the error correction electronics connected into the circuitry.
The above and below descriptions describe amplifier circuitry which is not symmetrical relative to the positive and negative power supply rails. This is for simplicity, and the same basic description could equally be applied to more or fully symmetric circuitry.